Method and apparatus for electro-optic delay generation of optical signals

ABSTRACT

An optical delay generator includes a waveguide made from electro-optically active material which contains a chirped distributed Bragg reflector. An electric field generated across the waveguide causes the index of refraction within the waveguide to change. A change in the index of refraction results in a change in the point at which light is reflected from the chirped distributed Bragg reflector within the waveguide, thus providing a controllable delay for optical pulses. Optical pulse position modulation is provided by using the optical delay generator to control the delay imparted on each pulse within a stream of equally-spaced optical pulses.

FIELD OF THE INVENTION

The present invention relates to the processing of optical signals and,more particularly, to delaying optical signals.

BACKGROUND OF THE INVENTION

Many satellite and terrestrial optical communication systems requiretransmission of analog optical signals. One mechanism for thetransmission of analog optical signals is through the use of some sortof pulse modulation, where a stream of optical pulses is modulated by ananalog signal. Pulse Position Modulation (PPM) is a well-knownmodulation technique for radio-frequency transmissions. It is also usedin analog optical communications. In PPM, a shift in the position ofeach pulse represents a sample of the original analog signal. Since thepulse repetition frequency (PRF) of the optical pulses must be greaterthan twice the bandwidth of the analog signal to correctly sample theanalog signal, PRFs for optical communications will be quite high. Forexample, an optical inter-satellite link designed to transmit waveformswith a bandwidth of 20 GHz requires a PRF of over 40 GHz.

The optical pulses within the stream should be of short duration, sinceit is well known in the art that PPM signal-to-noise ratio (SNR)performance improves as the pulse widths within the modulated pulsestream decrease. Pulse widths as short as 0.3 picoseconds may bedesirable for a PPM optical communication system. However, is also wellknown in the art that PPM performance will suffer if the shapes of theoptical pulses vary or the amplitudes of the pulses vary on apulse-to-pulse basis. Mode locking of a pulsed laser is a maturetechnique for producing equally spaced ultra-short identical pulses. Itwould be beneficial to use a mode-locked laser in a PPM communicationsystem if the equally-spaced pulses produced by the system could bemodulated without distortion.

Therefore, implementations of PPM for optical communications require amechanism for modulating the delays between extremely short opticalpulses within a pulse stream without modulating the shapes orpulse-to-pulse amplitudes of the pulses. Direct modulation of asemiconductor laser will appropriately modulate the delay between theoptical pulses generated by the laser. However, a directly modulatedsemiconductor laser generates relatively long pulses that result inlimited SNR performance. Pulse compression can be used on the longerpulses produced by the directly modulated semiconductor laser, butdevices to provide such compression are complex and cumbersome. Directmodulation of a semiconductor laser may also introduce amplitudemodulation or pulse reshaping of the individual time-shifted pulses,further limiting performance.

Pulse position modulation of extremely short optical pulses is alsoachieved by applying a pulse-to-pulse delay external to the source ofthe equally spaced optical pulses. That is, a method and apparatus areused that can receive a stream of optical pulses, change the pulse-topulse delay at the rate required for properly sampling the transmittedanalog signal, and further transmit the delayed pulses. It is known inthe art that one example of a pulse position modulator for opticalpulses consists of an optical delay line, such as a parallel slab oftransparent electro-optically active material. The refractive index ofthe electro-optically active material can be controllably varied by anapplied voltage, so that each pulse is controllably delayed upontraversing the electro-optically active material in accordance with theinstaneous voltage. However, such a modulator requires an undesirablylarge amount of electrical power, due to the relatively large voltagesrequired to modulate the refractive index of the material and thusmodulate the delay encountered by a pulse traversing the material.

Another example of a pulse position optical modulator relying upon theuse of electroptically active material is disclosed in U.S. Pat. No.3,961,841, issued Jun. 8, 1976 to Giordmaine. Giordmaine discloses adevice for optical pulse position modulation comprising a diffractiongrating in combination with an electro-optic prism and a lens. Thediffraction grating splits an incident light pulse into its frequencycomponents and the lens directs the components into the prism. Therefractive index change provided by the prism causes a phase shift inthe frequency components and thus a time shift in the optical pulse onceit is reconstructed by the diffraction grating. The device disclosed byGiordmaine provides the capability of modulating light pulses as shortas one picosecond. However, the maximum controllable delay is limited toa few picoseconds for a 3 picosecond pulse and further decreases forshorter pulses. Also, the multiplicity of optical elements such as thediffraction grating, lens, and prism increase the complexity andmanufacturing cost of the device.

A device for delaying optical pulses is disclosed in U.S. Pat. No.5,751,466, issued May 12, 1998 to Dowling et al and is shown in FIG. 1.Dowling discloses a photonic bandgap structure comprising a plurality ofcells 18A-18N of width d in which the refractive index varies. Therefractive index variation may be such that each cell comprises twolayers of materials with two different indices of refraction n₁ and n₂.If the widths of the two layers within each cell are λ/4n₁ and λ/4n₂where λ is the free space wavelength of the optical pulse to be delayed,a distributed Bragg reflector structure is created. According toDowling, the thickness and/or number of layers in the photonic bandgapstructure and/or their indices of refraction are selected to produce astructure with a transmission resonance center frequency and bandwidthcorresponding to the frequency and bandwidth of the optical pulse to bedelayed. By matching the transmission resonance to the optical pulse, acontrollable delay is imparted to the optical. pulse withoutsignificantly altering the optical signal.

The device disclosed by Dowling requires that the thickness of eachlayer in the device be approximately one-half the wavelength of theincident optical pulse to form the photonic bandgap structure. The delayimparted on an optical signal by transmission through the structure willdepend upon the number of layers and the indices of refraction withinthe layers. The structure can be thought of as essentially increasingthe length of the waveguide in which it is contained, thus providing thedesired delay. For example, Dowling discloses a simulation of a photonicbandgap structure that is 7 μm thick that provides a delay equivalent toan optical signal traveling through a 110 μm stucture, or a delay ofabout 0.4 picoseconds. Since the amount of delay from a single structureis relatively small, Dowling discloses that the structures can besuccessively coupled in a single device to provide additional delay. Ofcourse, this increases the overall size of the device.

Dowling also discloses that the delay provided by a photonic bandgapstructure can be varied by changing the indices of refraction within thelayers of the structure. One way to accomplish this is to fabricate atleast one of the layers from electro-optically active material. Anapplied voltage will then change the index of refraction in the layer towhich the voltage is applied. FIG. 1 shows a voltage means 15 thatapplies a voltage to one or more of the layers within the devicedisclosed by Dowling. Varying the voltage would vary the delay, thusproviding the controllable delay required for pulse position modulation.However, since the overall delay provided by photonic bandgap structureis relatively small, it would follow that the change of delay providedby electro-optically changing the indices of refraction would only besome fraction, typically 0.1% or less, of that relatively small delay.Again, this limitation could be overcome by coupling successivestructures, with a corresponding increase in the overall size of thestructure.

There exists a need for a high quality optical delay apparatus andmethod that provide large, controllable delays for short optical pulses.Moreover, the apparatus and method must be capable of providing therequired delay without substantially altering the pulse-to-pulseamplitude or shape of the pulses in the original pulse stream.Additionally, it is important for the delay generation to be implementedin a compact, lightweight apparatus that is compatible with otherintegrated systems.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provideapparatus and methods for optical delay generation.

It is another object of the present invention to provide apparatus andmethods for optical delay generation without causing pulse-to-pulseamplitude modulation or pulse reshaping of delayed optical pulses.

It is another object of the invention that the method and apparatusprovide optical delay that can be used for pulse position modulation.

These and other objects are provided according to the present inventionby transmitting optical pulses to be delayed into a waveguide meanscomprising electro-optically active material within which is formed achirped distributed Bragg reflector (C-DBR) oriented in the direction oflight propagation within the waveguide means. The chirped distributedBragg reflector reflects light at different wavelengths at differentpoints within the waveguide. An electric field generator generates andcontrols an electric field applied across the waveguide in a directionperpendicular to the direction of propagation. Changes in the electricfield intensity cause changes in the index of refraction within thewaveguide means, thus changing the point at which the optical pulsesreflect from the chirped distributed Bragg reflector and are transmittedout of the waveguide means. Thus, optical delay generation isaccomplished by controlling the intensity of the electric field acrossthe chirped distributed Bragg reflector.

In a first specific embodiment of the present invention, the waveguidemeans comprises a straight waveguide constructed from electro-opticallyactive material, such as lithium niobate, sandwiched between a topconductor and a bottom conductor. A chirped distributed Bragg reflectoris formed in the waveguide by quasiperiodically corrugating thewaveguide walls. A voltage source is connected to the top conductor andthe bottom conductor such that a voltage between the two is created. Thevoltage causes an electric field to be generated across the chirpeddistributed Bragg reflector, thus changing the index of refraction asthe voltage changes. An alternate embodiment uses a tapered waveguide inwhich the waveguide walls are periodically corrugated.

In a second embodiment of the present invention, the electro-opticallyactive material used in the waveguide means comprises a semiconductorchirped distributed Bragg reflector structure with excitonic band justabove the photon energy structure. The chirped distributed Braggreflector is formed by the individual layers of semiconductor material.The refractive index and thickness of each layer vary from its neighborso as to provide the quasiperiodic variation in refractive indexrequired to form a chirped distributed Bragg reflector.

The chirped distributed Bragg reflector of an alternative embodiment ofthe present invention comprises an apodized chirped distributed Braggreflector. Apodization of the chirped distributed Bragg reflectorreduces the oscillations in the group delay of the optical pulse thatwould result if the optical pulse were reflected by a linearly chirpeddistributed Bragg reflector. Hence, distortion of optical pulses isreduced.

Reflection of optical pulses from a chirped distributed Bragg reflectorresults in broadening of the optical pulses due to an acquired chirp.Therefore, in another embodiment of the present invention, thetime-delayed pulses output from the delay generator are passed through adispersion compensating fiber, which provides correction for theacquired chirp.

The present invention is used to provide optical pulse positionmodulation for an analog signal. A stream of equally-spaced opticalpulses is transmitted into a waveguide containing a chirped distributedBragg reflector. The analog signal controls a modulation means thatgenerates an electric field across the waveguide. The modulation meanscontrols the intensity of the electric field and thus the delay providedby the waveguide. Each optical pulse in the stream of optical pulses isreflected by the chirped distributed Bragg reflector and acquires adelay corresponding to the analog signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) shows a photonic bandgap structure used for delayingoptical pulses.

FIG. 2 shows a schematic representation of an optical delay generator inaccordance with the present invention comprising a waveguide with achirped distributed Bragg reflector and an optical circulator fordirecting pulses into and out of the waveguide.

FIG. 3A shows a graphical representation of the variation in refractiveindex required for a chirped distributed Bragg reflector.

FIG. 3B shows a graphical representation of the variation in refractiveindex required for an apodied chirped distributed Bragg reflector.

FIG. 4 shows an alternative embodiment of the present inventioncomprising a straight waveguide where the core width variesquasiperiodically to create a chirped distributed Bragg reflector.

FIG. 5 shows another embodiment of the present invention comprising atapered waveguide where the core width varies periodically to create achirped distributed Bragg reflector.

FIG. 6 shows another embodiment of the present invention comprising awaveguide with a plurality of electrodes where variations in theelectric field generated by the separate electrodes create a chirpeddistributed Bragg reflector.

FIG. 7 shows an embodiment of the present invention comprising asemiconductor chirped distributed Bragg reflector structure where layersof varying width and refractive index form the chirped distributed Braggreflector.

FIG. 8 shows another embodiment of the present invention where anoptical signal reflected from a chirped distributed Bragg reflector isdirected into a dispersion compensating filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. In the drawings, the thicknesses of layersand regions are exaggerated for clarity.

Referring now to FIG. 2, a schematic representation of apparatus andmethods for optical delay generation is shown. Referring to FIG. 2, awaveguide means 100 receives light pulses 31 a at times t_(n) andimparts a delay Δt_(n) on each pulse to thereby produce reflected lightpulses 31 b at times t_(n)+Δt_(n). Preferably, light pulses 31 b aresimply delayed versions of light pulses 31 a such that the delayedpulses 31 b replicate the original light pulses 31 a in terms of pulseshape and have not acquired any pulse-to-pulse amplitude modulation.

Still referring to FIG. 2, the waveguide means 100 comprises one or morelayers of electro-optically active material 107 with a varyingrefractive index. The present invention requires that the variations inthe refractive index within the electro-optically active material form achirped distributed Bragg reflector (C-DBR). The C-DBR reflects anoptical signal of a specific wavelength after the optical signal hastraveled a certain distance z within the waveguide means, as indicatedby line 108. The construction and features of the C-DBR will bedescribed in more detail below.

The index of refraction of electro-optically active material changeswhen an electrical field is applied. In FIG. 2, a means for generatingan electric field across the electro-optically active material 107 isshown as being provided by an upper electrode 105 and a lower electrode106 connected to a voltage source 109. As the electric field between thetwo electrodes changes, the index of refraction within theelectro-optically active material will change. The change of therefractive index is generally proportional to the magnitude of theelectric field and is represented by dn=(dn/dE)dE. As will be describedbelow, changes in the refractive index result in changes in the distancein which an optical signal travels in the C-DBR before it is reflected,resulting in changes in the amount of delay applied to an opticalsignal.

Still referring to FIG. 2, the optical delay generator apparatus alsoincludes means for directional coupling the optical signals into and outof the waveguide means A fiber optic circulator 30 or other conventionalcoupling means may be used. Ideally, the circulator or other couplingmeans is low loss and will not cause any pulse reshaping orpulse-to-pulse amplitude modulation.

Electro-optically active materials are well known in the art. However,for use in the present invention, the electro-optically active materialmust be such that a C-DBR can be formed within it. Also, the dn/dEfactor should be as large as possible, so that the magnitude of theelectric field can be kept as small as possible. Materials that providesuch characteristics include lithium niobate (LiNbO₃), lithium tantalate(LiTaO₃), and lithium niobate doped with titanium. The electro-opticallyactive material may also comprise an electro-refractive semiconductorCDBR structure. A semiconductor C-DBR structure comprises several verythin layers of materials having different refractive indices. Each layercomprises semiconductor material known to exhibit an excitonic band justabove the photon energy. With such material, the electrical fieldmagnitude required to produce the desired refractive index change isreduced.

A periodic (or quasiperiodic) fluctuation in the core refractive indexof optical media results in a “Bragg grating” or a “distributed Braggreflector.” The pattern of fluctuations behaves as a spectrallyselective reflector for electromagnetic radiation. The reflection of adistributed Bragg reflector reaches its maximum at the wavelength λsatisfying the Bragg condition:

 β(λ)=π/Λ  (1)

where β(λ) is the wave number at the given wavelength and Λ is theperiod of modulation of the distributed Bragg reflector.

The present invention requires that the distributed Bragg reflector bequasiperiodic instead of periodic. That is, the period of the refractiveindex variation (i.e., the linear distance, between successive peaks andvalleys of the refractive index profile) is not a constant, but insteadchanges in a predetermined fashion along the propagation axis of thedistributed Bragg reflector. The propagation axis of the DBR is thedirection in which the incident light travels. Such a Bragg reflector isreferred to as a “chirped” distributed Bragg reflector. Preferably, thepresent invention utilizes a quasiperiodic variation in the refractiveindex in which the period increases or decreases as an approximatelylinear function of position along the propagation axis, resulting in alinearly chirped distributed Bragg reflector. FIG. 3A shows a “chirped”variation of the refractive index n as a function of position z alongthe propagation axis.

As indicated above, a chirped distributed Bragg reflector is created inelectro-optically active material by modulating the refractive indexwithin the material. Creation of chirped distributed Bragg reflectors iswell known in the art. U.S. Pat. No. 4,953,939, issued Sep. 4, 1990 toR. Epworth, describes several methods for creating chirped distributedBragg reflectors within optical fibers. All of the methods disclosed byEpworth describe the creation of quasi-periodic corrugations within thewalls of the optical fiber, where the wall of the fiber is the interfacebetween a core and a cladding within the fiber. These methods forcreating a chirped distributed Bragg reflector within optical fiberswould also be used to create chirped distributed Bragg reflectors withinstraight waveguides made from electro-optically active material such aslithium niobate or lithium tantalate as used in some embodiments of thepresent invention. It is also known in the art that a chirpeddistributed Bragg reflector will result when a tapered waveguidecontains periodic corrugations on its walls, where the period of thecorrugations roughly corresponds to the Bragg wavelength.

FIG. 4 illustrates an embodiment of the present invention which uses astraight waveguide. In FIG. 4, a straight waveguide 120 has a core 124and a cladding 125. The waveguide 120 has been constructed such thatwidth of the core 125 varies in a quasiperiodic fashion so as to createa chirped distributed Bragg reflector. The waveguide 120 is sandwichedby a top electrode 123 and a bottom electrode 122, which are connectedto a voltage source 129 to generate a voltage and thus an electric fieldacross the waveguide. Light pulses 31 a enter one end of the waveguide120 and are reflected by the chirped distributed Bragg reflector withinthe waveguide 120. An alternative embodiment is shown in FIG. 5, where atapered waveguide 130 with a core 134 and a cladding 135 is constructedsuch that the width of the core 134 varies in a periodic fashion.

A method known in the art as polling also creates a chirped distributedBragg reflector. In polling, a quasi-periodic DC electric field isapplied along a waveguide constructed from electro-optically activematerial. The quasi-periodic variations in the DC electric field causequasiperiodic variations in the local index of refraction, resulting ina chirped distributed Bragg reflector. FIG. 6 illustrates an embodimentof the present invention that uses polling. A waveguide 140 containselectro-optically active material 144 sandwiched between a plurality oftop electrodes 142 and a bottom conductor 143. A voltage source 146connects to the plurality of top electrodes 142 and the bottom conductor143 so as to provide an electric field between each top electrode 142and the bottom conductor 143. The voltage source 146 controls thevoltage at each electrode so as to provide a voltage that is the sum ofa two voltage components as shown below:

V _(i) =V _(uniform) +V _(poll)(i)

where V_(i) is the total applied voltage at the ith electrode,V_(uniform) is a voltage to be applied uniformly across the waveguideand V_(poll)(i) is a voltage to be applied at the ith electrode toachieve polling. The first voltage component provides a uniform electricfield that would result in a uniform change in the index of refractionalong the waveguide 140 in the absence of the second voltage component.Thus, the first voltage component controls the delay provided by thewaveguide. The second voltage component provides an electric field thatvaries quasi-periodically along the length of the waveguide so as toresult in a quasi-periodic variation in the index of refraction withinthe waveguide 140 in the absence of the first voltage component. Thus,the second voltage component creates a chirped distributed Braggreflector within the waveguide.

If a semiconductor C-DBR structure is used to provide the wave guidemeans of the present invention, the chirped distributed Bragg reflectoris formed by controlling the refractive index within the individuallayers of the semiconductor structure. One such semiconductor structurecan be formed from alternating layers of low refractive index aluminumarsenide and high refractive index aluminum gallium arsenide. FIG. 7.shows an embodiment of the present invention using a semiconductorstructure with alternating low and high refractive index layers. In FIG.7, alternating layers of low refractive index material (151A . . . 151N)and high refractive index material (152A . . . 152N) are used to form anelectro-optically active waveguide. Each alternation between a low indexlayer and high index layer is a single refractive index period. Thethickness of the alternating layers (d_(A) . . . d_(N)) is increased ina quasi-periodic fashion to provide a linearly increasing change in eachrefractive index period. A uniform electric field, controlled by avoltage source 159 and applied across the structure by a first electrode153 and a second electrode 154, controls the amount of delay provided bythe structure.

A chirped distributed Bragg reflector can also bc formed within a singlelayer of electro-optically active material by controlling the doping ofthat layer of material. For example, the refractive index of a lithiumniobate waveguide can be modulated by doping the lithium niobateperiodically or quasiperiodically with titanium. A chirped distributedBragg reflector will result if the periodicity of the doping satisfiesthe Bragg condition described above.

To further describe the apparatus and method of the present invention,reference is made to the straight waveguide embodiment of the presentinvention, as shown in FIG. 4. In this case, the wave number satisfyingthe Bragg condition is approximately given by β=2 π/nλ, where n is theeffective refraction index. In a linearly chirped DBR,

Λ(z)=Λ₀ +Λ′z  (3)

where Λ′ is the chirp parameter, and z is the classical turning pointfor a given wavelength. The chirp parameter Λ′=ΔΛ/L where ΔΛ is thechange in the C-DBR period across a C-DBR structure of total length L.The turning point z is found from the Bragg condition equation:

nλ=2 π/β=2π/(π/ζ(z))=2(Λ₀ +Λ′z)  (4)

The group delay introduced by a chirped distributed Bragg reflector isapproximately

t _(d)=2nz/c  (5)

where c is the speed of light. This equation demonstrates that the groupdelay can be changed by changing the effective refractive index of thewaveguide. If the index of refraction is changed as a result of theapplication of an electric field to an electro-optically activematerial, the group delay will be changed as shown in the equationbelow:

dt _(d)=(2dn/cΛ′)(nλ−Λ ₀)≈2(Λ₀ cΛ′)dn  (6)

The change in the index of refraction will be nearly instantaneous tothe change in the electric field, thus providing the capability toquickly change the delay provided by the present invention. Changes inthe electric field, however, will be limited by the means used to applyand control the electric field. Control electronics, impedance ofelectrodes, and other electrical effects may limit the speed at whichthe present invention operates, but the present invention itself canprovide nearly instantaneous change in applied delay. Electro-opticalmodulators that do not use a chirped distributed Bragg reflector havebeen demonstrated to operate at 50 GHz, so the present invention is alsoexpected to provide operation up to 50 GHz or higher.

The classical description of group delay presented above is only anapproximation. For example, it does not account for oscillations in thegroup delay that result from a linearly chirped grating. However, acorrectly engineered chirp will eliminate the resonances and a lineargroup delay dependence is realized. One such chirped grating that willreduce the oscillations is an apodized chirped distributed Braggreflector. In an apodized chirped distributed Bragg reflector, theamplitude of the chirped index of refraction is tapered from a minimumto a maximum and then back to a minimum within the chirped distributedBragg reflector. FIG. 3B illustrates the refractive index variation foran apodized chirped distributed Bragg reflector.

Proper choice of the length and chirp of the C-DBR provides the desireddelay range for optical pulses of a certain duration, and thus providesthe capability for pulse position modulation. To illustrate thecalculations used, a delay generator based on a straight waveguide witha linearly varying C-DBR period is used, as shown in FIG. 4. Theelectro-optically active material used in the waveguide is LiNbO₃,although, as indicated previously, other materials may be used. Anyunwanted resonances in the group delay are eliminated by the procedurepreviously described.

The average C-DBR period is determined by the wavelength of the opticalsource and is calculated from the equation for the Bragg condition. Foran optical wavelength λ=1.55 mm and the LiNbO₃ index of refractionn_(e)=2.2, the average C-DBR period Λ₀=n_(e) λ/2=1.75 mm. The requireddifferential group delay dt_(p) determines the chirp for a givendifferential index of refraction dn. It is well known in the art thatfor LiNbO₃, the differential index of refraction as a function of theapplied electric field is derived from the equation:

dn=r ₃₃ n _(e) ³ dE _(z)  (7)

where the electro-optic coefficient r₃₃=30.8×10⁻¹² m/V. Assuming a 3mm-wide waveguide and 5 V of applied voltage, the differential index ofrefraction dn=5.5×10⁻⁴. If a group delay dt_(p)=5 ps is required, thechirp of the C-DBR Λ′=10⁻² μm/cm.

The delay generator must have sufficient bandwidth to reflect shortoptical pulses. It is well known in the art that the approximatespectral width of a chirped distributed Bragg reflector

Δλ_(C-DBR)=2Λ′L/n _(e),  (8)

where L is the total length of the distributed Bragg reflector. Toreflect the full spectrum of the optical pulses transmitted into thedelay generator containing the distributed Bragg reflector, the lengthof the reflector L must be greater than Δλn_(e)/2Λ′. For example, thespectral width of a Gaussian pulse of t=0.3 ps duration and wavelengthλ=1.55 mm is Δλ_(t)=0.44λ²/ct=12 nm. For the delay generator usingLiNbO₃ previously described, the length of the DBR must exceedL=Δλ_(t)n_(e)/2Λ′≈1.3 cm. Electro-optical waveguide modulators with L=1cm and longer are common in the art.

The present invention provides optical pulse position modulation byusing a modulation means to control the optical delay provided by thechirped distributed Bragg reflector as shown in FIG. 2. For pulseposition modulation, a stream of equall spaced optical pulses istransmitted into the input of an optical circulator 30. The opticalcirculator than sends these pulses into the waveguide 100 comprisingelectro-optically active material and containing a chirped distributedBragg reflector. An analog signal to be pulse positioned modulatedcontrols the voltage source 109. The voltage source 109 controls anelectric field within the waveguide 100, and thus controls the delayimparted on each optical pulse 31 a transmitted into the waveguide 100.Each optical pulse 31 a will acquire a delay corresponding to the delayrequired to pulse position modulate the analog signal, and will bereflected out of the waveguide 100 and transmitted out of the circulator30 as a delayed pulse 31 b.

The optical pulses reflected by the described delay generator arebroadened due to the chirp imposed by the C-DBR. This broadening or“chirping” of the pulses may reduce the performance of a pulse positionmodulation system provided by the present invention. However, this chirpcan be removed by passing the chirped pulses through a dispersioncompensating fiber as is customary in the art. FIG. 8 demonstrates oneway in which the acquired chirp can be removed. In FIG. 8, the delayedoptical pulses output by the coupler 31 b are sent through a dispersioncompensating fiber 32. Dispersion compensating filters are well known inthe art. The filtered pulses 31 c output by the dispersion compensatingfilter 32 should match the optical pulses 31 a input to the system inamplitude and pulse width.

Pulse position modulation provided by the present invention minimizespulse-to-pulse amplitude or shape modulation. As shown in the examplepreviously described, the change in the refraction index,dn=5.5×10⁻⁴<<n_(e)=2.2 and the accompanying shift of the C-DBR band toachieve the desired group delay dt_(d)=5 ps are very small, that isdλ/dλ_(t)∝dn/n_(e)<<1. Hence, attenuation of the optical signal due tothe spectral changes in frequency response of the waveguide resultingfrom the changes in the refraction index should be negligible.

From the foregoing description, it will be apparent that the presentinvention has a number of advantages, some of which have been describedabove, and others of which are inherent in the embodiments of theinvention described above. Also, it will be understood thatmodifications can be made to the optical delay generator and method forperforming pulse position modulation described above without departingfrom the teachings of subject matter described herein. As such, theinvention is not to be limited to the described embodiments except asrequired by the appended claims.

What is claimed is:
 1. An optical delay generator comprising: awaveguide comprising at least one layer of electro-optically activematerial; a chirped distributed Bragg reflector formed within said atleast one layer of electro-optically active material, said chirpeddistributed Bragg reflector having a direction of propagation; and, ameans for applying an electric field across said electro-opticallyactive material, said electric field applied in a direction normal tosaid direction of propagation of said chirped distributed Braggreflector.
 2. The optical delay generator of claim 1 wherein saidwaveguide comprises: a cladding, and a core within said cladding; saidcore having walls with corrugations to form said chirped distributedBragg reflector.
 3. The optical delay generator of claim 2 wherein saidwaveguide is tapered and said walls have periodic corrugations.
 4. Theoptical delay generator of claim 2 wherein said waveguide is of constantwidth and said walls have quasiperiodic corrugations.
 5. The opticaldelay generator of claim 1 wherein said means for applying an electricfield applies an electric field that varies in intensityquasiperiodically in a direction parallel to said direction ofpropagation of said chirped distributed Bragg reflector, whereinvariations in said electric field create said chirped distributed Braggreflector.
 6. The optical delay generator of claim 1 wherein saidwaveguide comprises: a plurality of layers of near-resonantsemiconductor material, said layers forming said chirped distributedBragg reflector.
 7. The optical delay generator of claim 1 wherein saidchirped distributed Bragg reflector has a length sufficient to reflect afull spectrum of optical pulses transmitted into said waveguide means.8. The optical delay generator of claim 1 wherein said chirpeddistributed Bragg reflector is an apodized chirped distributed Braggreflector.
 9. The optical delay generator of claim I wherein said meansfor inducing an electric field comprises: a first electrode located onone side of said at least one layer of electro-optically activematerial; a second electrode located on a side of said at least onelayer of electro-optically active material opposite the location of saidfirst electrode; and, a voltage source, said voltage source connected tosaid first electrode and said second electrode, wherein said voltagesource generates a voltage between said first electrode and said secondelectrode.
 10. The optical delay generator of claim 1 wherein said meansfor inducing an electric field comprises: a common electrode located onone side of said at least one layer of electro-optically activematerial; a plurality of electrodes located on a side of said at leastone layer of electro-optically active material opposite the location ofsaid common electrode; and, a voltage source, said voltage sourceconnected to said common electrode and said plurality of electrodeswherein said voltage source generates a voltage between said commonelectrode and said plurality of electrodes.
 11. The optical delaygenerator of claim 5 wherein said means for inducing an electric fieldcomprises: a common electrode located on one side of said at least onelayer of electro-optically active material; a plurality of electrodeslocated on a side of said at least one layer of electro-optically activematerial opposite the location of said common electrode; a voltagesource, said voltage source connected to said common electrode and saidplurality of electrodes wherein said voltage source generates acontrollable voltage between said common electrode and each electrode insaid plurality of electrodes, wherein said controllable voltagecomprises a first voltage uniformly applied at each electrode and asecond voltage that varies with each electrode in said plurality ofelectrodes, said second voltage creating said variations in saidelectric field.
 12. The optical delay generator of claim 1 wherein saidelectro-optically active material is lithium niobate, lithium tantalate,lithium niobate doped with titanium, aluminum arsenide, or aluminumgallium arsenide.
 13. An optical delay generator comprising: a waveguidecomprising at least one layer of electro-optically active material; achirped distributed Bragg reflector formed within said at least onelayer of electro-optically active material, said chirped distributedBragg reflector having a direction of propagation; and, a voltage sourcewhich induces an electric field across said electro-optically activematerial in a direction normal to said direction of propagation of saidchirped distributed Bragg reflector.
 14. The optical delay generator ofclaim 13 wherein said waveguide comprises: a cladding, and a core withinsaid cladding, said core having walls with corrugations to form saidchirped distributed Bragg reflector.
 15. The optical delay generator ofclaim 14 wherein said waveguide is tapered and said walls have periodiccorrugations.
 16. The optical delay generator of claim 14 wherein saidwaveguide is of constant width and said walls have quasiperiodiccorrugations.
 17. The optical delay generator of claim 13 wherein saidvoltage source induces an electric field that varies in intensityquasiperiodically in a direction parallel to said direction ofpropagation of said chirped distributed Bragg reflector, whereinvariations in said electric field create said chirped distributed Braggreflector.
 18. The optical delay generator of claim 13 wherein saidwaveguide comprises: a plurality of layers of near-resonantsemiconductor material, said layers forming said chirped distributedBragg reflector.
 19. The optical delay generator of claim 13 whereinsaid chirped distributed Bragg reflector has a length sufficient toreflect a full spectrum of optical pulses transmitted into saidwaveguide means.
 20. The optical delay generator of claim 13 whereinsaid chirped distributed Bragg reflector is an apodized chirpeddistributed Bragg reflector.
 21. The optical delay generator of claim 13wherein said voltage generator comprises: a first electrode located onone side of said at least one layer of electro-optically activematerial; a second electrode located on a side of said at least onelayer of electro-optically active material opposite the location of saidfirst electrode; and, a voltage source, said voltage source connected tosaid first electrode and said second electrode, wherein said voltagesource generates a voltage between said first electrode and said secondelectrode.
 22. The optical delay generator of claim 13 wherein saidvoltage generator comprises: a common electrode located on one side ofsaid at least one layer of electro-optically active material; aplurality of electrodes located on a side of said at least one layer ofelectro-optically active material opposite the location of said commonelectrode; and, a voltage source, said voltage source connected to saidcommon electrode and said plurality of electrodes wherein said voltagegeneration means generates a voltage between said common electrode andsaid plurality of electrodes.
 23. The optical delay generator of claim17 wherein said voltage source comprises: a common electrode located onone side of said at least one layer of electro-optically activematerial; a plurality of electrodes located on a side of said at leastone layer of electro-optically active material opposite the location ofsaid common electrode; a voltage source, said source connected to saidcommon electrode and said plurality of electrodes wherein said sourcegenerates a controllable voltage between said common electrode and eachelectrode in said plurality of electrodes, wherein said controllablevoltage comprises a first voltage uniformly applied at each electrodeand a second voltage that varies with each electrode in said pluralityof electrodes, said second voltage creating said variations in saidelectric field.
 24. The optical delay generator of claim 13 wherein saidelectro-optically active material is lithium niobate, lithium tantalate,lithium niobate doped with titanium, aluminum arsenide, or aluminumgallium arsenide.